IPv4 Subnets, Netmasks, and CIDR
CIDR notation writes an IPv4 network as an address followed by the number of network bits, such as 192.0.2.0/24. It is shorter and less error-prone than writing a dotted netmask alone.
Prefix lengths
| Prefix | Netmask | Addresses | Usable hosts* |
|---|---|---|---|
/30 |
255.255.255.252 |
4 | 2 |
/29 |
255.255.255.248 |
8 | 6 |
/28 |
255.255.255.240 |
16 | 14 |
/24 |
255.255.255.0 |
256 | 254 |
/16 |
255.255.0.0 |
65,536 | 65,534 |
*Traditional host counts exclude the network and broadcast addresses. Point-to-point links and IPv6 use different conventions.
Finding a range
For 192.0.2.37/27, the block size is 32 because /27 leaves five host bits. The ranges begin at .0, .32, .64, and so on. Therefore:
- Network:
192.0.2.32 - Usable range:
192.0.2.33through192.0.2.62 - Broadcast:
192.0.2.63
The address 192.0.2.37 is inside that range and is usable.
Practical checks
Use a calculator for complex plans, then verify assumptions with the host or network device. A configuration such as 192.0.2.20/24 means the host considers 192.0.2.0/24 local; it does not automatically configure a gateway or DNS server.
Common mistakes
- Assigning the network or broadcast address to a host.
- Using overlapping subnets on connected interfaces.
- Treating a prefix length as a host count.
- Forgetting that firewall rules and routes still control traffic.
Return to Linux Network Configuration with Netplan to apply these calculations safely.